We report a case of possible retroperitoneal metastasis of breast cancer successfully treated with oral S-1 and cyclophosphamide therapy after docetaxel and cyclophosphamide (TC) therapy. A 57-year-old woman with a history of bilateral breast cancer showed an increase in tumor markers during treatment with oral anastrozole as postoperative adjuvant therapy 4 years after her second cancer surgery. After careful examination, the patient was diagnosed as having multiple bone metastases and her medication was changed to oral letrozole. After 3 months, the patient developed left back pain and was referred to our hospital. CT scanning showed an enhanced mass in the region from the left perirenal and posterior pararenal spaces to the left psoas major muscle and the anterior aspect of the left iliacus muscle, suggesting retroperitoneal metastasis. TC therapy was performed and, as a result, tumor markers decreased and the mass disappeared on CT imaging. After discontinuation of TC therapy, the tumor markers increased again, following which oral S-1 and cyclophosphamide therapy were administered, and the tumor markers decreased. At the time of this writing, the patient is still undergoing therapy, and no recurrence has been observed. We concluded that oral S-1 and cyclophosphamide therapy were useful in the present case and were associated with few adverse effects.